In mobile communications, automatic repeat request (“ARQ”) is applied to downlink data from a radio communication base station apparatus (hereinafter “base station”) to a radio communication mobile station apparatus (hereinafter “mobile station”). That is, the mobile station feeds back a response signal showing an error detection result of downlink data to the base station. The mobile stations performs CRC (Cyclic Redundancy Check) check for uplink data, and, if CRC=OK (no error), feeds back an ACK (ACKnowledgment), and, if CRC=NG (error present), feeds back a NACK (Negative ACKnowledgment), as a response signal to the mobile station. This response signal is transmitted to the base station using an uplink control channel, for example, a PUCCH (Physical Uplink Control Channel).
Further, the base station transmits control information for reporting a resource allocation result of downlink data to the mobile station. This control information is transmitted to the mobile station using downlink control channels including L1/L2 CCHs (L1/L2 Control Channels). Each L1/L2 CCH occupies one or a plurality of CCEs (Control Channel Elements) according to the coding rate for control information. If the L1/L2 CCH for reporting control information of a coding rate 2/3 occupies one CCE, the L1/L2 CCH for reporting control information of a coding rate 1/3 occupies two CCEs, the L1/L2 CCH for reporting control information of a coding rate 1/6 occupies four CCEs, and the L1/L2 CCH for reporting control information of a coding rate 1/12 occupies eight CCEs. Further, when one L1/L2 CCH occupies a plurality of CCEs, one L1/L2 CCH occupies a plurality of consecutive CCEs. The base station generates a mobile station-specific L1/L2 CCH, allocates CCEs that should be occupied by the L1/L2 CCH according to the number of CCEs required for control information, and maps the control information to physical resources corresponding to the allocated CCEs, to transmit the control information.
Further, to eliminate the need for signaling for reporting PUCCHs, which are used to transmit response signals, from the base station to mobile stations and to use downlink resources efficiently, studies are underway to associate CCEs with PUCCHs one by one (see Non-Patent Document 1). According to this association, each mobile station is able to identify the PUCCH to use to transmit a response signal from the mobile station, from CCEs corresponding to physical resources to which control information for the mobile station is mapped. Consequently, each mobile station maps a response signal from the mobile station, to the physical resource based on the CCE corresponding to the physical resource to which control information for the mobile station is mapped. If a CCE corresponding to a physical resource to which control information directed to a mobile station is mapped to is CCE #0, the mobile station decides PUCCH #0 associated with CCE #0 to be the PUCCH for the mobile station. Further, if the CCEs corresponding to physical resources to which control information directed to a mobile station is mapped, are CCE #0 to CCE#3, the mobile station decides PUCCH #0 associated with CCE #0, which is the smallest number among CCE #0 to CCE #3, to be the PUCCH for the mobile station. If the CCEs corresponding to physical resources to which control information directed to a mobile station is mapped, are CCE #4 to CCE #7, the mobile station decides PUCCH #4 associated with CCE #4, which is the smallest number among CCE #4 to CCE #7, to be the PUCCH for the mobile station.
Further, as shown in FIG. 1, studies are underway to perform code-multiplexing by spreading a plurality of response signals from a plurality of mobile stations using ZAC sequences and Walsh sequences (see Non-Patent Document 2). In FIG. 1, [W0, W1, W2, W3] represent a Walsh sequence of a sequence length of 4. As shown in FIG. 1, in a mobile station, a response signal of ACK or NACK is subject to first spreading to one symbol in the time domain by a ZAC sequence (with a sequence length of 12) in the frequency domain. Next, the mobile station associates the response signal after the first spreading with W0 to W3 and performs an IFFT (Inverse Fast Fourier Transform). By this IFFT, the response signal spread in the frequency domain is converted to a ZAC sequence of a sequence length of 12 in the time domain. Then, the signal after the IFFT is secondly spread using Walsh sequences (with a sequence length of 4). That is, one response signal is assigned to each of four symbols S0 to S3. Likewise, other mobile stations spread response signals using a ZAC sequence and a Walsh sequence. Different mobile stations use ZAC sequences of different amount of cyclic shift in the time domain or use different Walsh sequences. Here, the sequence length of a ZAC sequence in the time domain is 12, so that it is possible to use twelve ZAC sequences of amounts of cyclic shift “0” to “11” generated from the same ZAC sequence. Also, the sequence length of a Walsh sequence is 4, so that it is possible to use four varying Walsh sequences. Consequently, in an ideal communication environment, it is possible to code-multiplex response signals from maximum forty-eight (12×4) mobile stations.
Further, as shown in FIG. 1, studies are underway to code-multiplex a plurality of reference signals (pilot signals) from a plurality of mobile stations (see Non-Patent Document 2). As shown in FIG. 1, when three symbols of a reference signal R0, R1 and R2 are generated from a ZAC sequence (with a sequence length of 12), the ZAC sequence is subject to an IFFT (Inverse Fast Fourier Transform) in association with an orthogonal sequence, for example, a Fourier sequence [F0, F1, F2] of a sequence length of 3. This IFFT makes it possible to acquire a ZAC sequence of a sequence length 12 in the time domain. Then, the signal after the IFFT is spread using the orthogonal sequence [F0, F2]. That is, one reference signal (a ZAC sequence) is allocated to three symbols R0, R1 and R2. Likewise, one reference signal (a ZAC sequence) is allocated to three symbols R0, R1 and R2 in other mobile stations. Different mobile stations use ZAC sequences of different amount of cyclic shift in the time domain or use different orthogonal sequences. Here, the sequence length of a ZAC sequence in the time domain is twelve, so that it is possible to use twelve ZAC sequences of amounts of cyclic shift “0” to “11” generated from the same ZAC sequence. Further, the sequence length of an orthogonal sequence is three, so that it is possible to use three varying orthogonal sequences. Consequently, in an ideal communication environment, it is possible to code-multiplex the maximum thirty-six (12×3) reference signals from the mobile stations.
Then, as shown in FIG. 1, one slot is composed of seven symbols, S0, S1, R0, R1, R2, S2 and S3.
Here, the cross-correlation between ZAC sequences between varying amounts of cyclic shift generated from a single ZAC sequence, is approximately zero. Consequently, in an ideal communication environment, correlation processing in the base station makes it possible to separate a plurality of response signals spread by ZAC sequences of varying amounts of cyclic shift (the amounts of cyclic shift 0 to 11) and code-multiplexed, almost without inter-code interference in the time domain.
However, a plurality of response signals from a plurality of mobile stations do not all arrive at the base station at the same time due to the difference of transmission timings between mobile stations, influence of multipath delayed waves and so on. For example, when the transmission timing of a response signal spread by the ZAC sequence of the amount of cyclic shift “0” is delayed from the correct transmission timing, the correlation peak of the ZAC sequence of the amount of cyclic shift “0” may appear in the detection window for the ZAC sequence of the amount of cyclic shift “1.” Further, when there is a delayed wave in a response signal spread by a ZAC sequence of the amount of cyclic shift “0,” an interference leakage due to that delayed wave may appear in the detection window for the ZAC sequence of the amount of cyclic shift “1.” Accordingly, in these cases, the ZAC sequence of the amount of cyclic shift “1” is interfered with the ZAC sequence of the amount of cyclic shift “0.” Consequently, in these cases, the separation performance degrades between a response signal spread by the ZAC sequence of the amount of cyclic shift “0” and a response signal spread by the ZAC sequence of the amount of cyclic shift “1.” Therefore, if ZAC sequences of adjacent amounts of cyclic shift are used, the separation performance of response signals may degrade.
Therefore, conventionally, if a plurality of response signals are code-multiplexed by spreading of ZAC sequences, a cyclic shift interval (i.e. the difference between the amounts of cyclic shift) is provided between ZAC sequences, to an extent that does not cause inter-code interference between ZAC sequences. For example, when the cyclic shift interval between ZAC sequences is 2, only six ZAC sequences of amounts of cyclic shift “0,” “2,” “4,” “6,” “8” and “10” are used for the first spreading of a response signal among twelve ZAC sequences of cyclic shift values “0” to “11.” Therefore, if a Walsh sequence of a sequence length of 4 is used for second spreading of a response signal, it is possible to code-multiplex response signals from maximum twenty-four (6×4) mobile stations.
However, as shown in FIG. 1, the sequence length of an orthogonal sequence to use to spread a reference signal is 3, and therefore only three varying orthogonal sequences can be used to spread a reference signal. Accordingly, when a plurality of response signals are separated using the reference signals shown in FIG. 1, it is possible to code-multiplex only response signals from maximum eighteen (6×3) mobile stations. Consequently, three Walsh sequences among four Walsh sequences of the sequence length of 4 are enough, and any one of Walsh sequences is not used.
Here, as described above, when a L1/L2 CCH occupies a plurality of CCEs, a mobile station transmits a response signal using the PUCCH associated with the smallest CCE number among a plurality of CCEs, and therefore PUCCHs associated with CCEs other than the CCE of the smallest number are not used and become useless. If an L1/L2 CCH occupies eight CCEs, CCE #0 to CCE #7, PUCCH #0 alone associated with the smallest CCE number #0 is used to transmit a response signal and PUCCHs #1 to #7 are not used, and therefore physical resources for PUCCHs #1 to #7 become useless. Also, to improve a rate of arrival of a response signal to the base station, when a mobile station transmits an identical response signal over a plurality of subframes a plurality of times in repetition, that is, when a mobile station repeats transmitting a response signal, a waste of physical resources for response signals increases depending on the number of times the response signals are transmitted.
To reduce a waste of physical resources involved in repeating transmitting a response signal, a technique of defining in advance subframes allowing repetition transmissions and subframes not allowing repetition transmissions, and preparing physical resources for repeating a response signal only to downlink data transmitted in specific subframes (see Non-Patent Document 3).    Non-Patent Document 1: Implicit Resource Allocation of ACK/NACK Signal in E-UTRA Uplink(ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-0724 39.zip)    Non-Patent Document 2: Multiplexing capability of CQIs and ACK/NACKs form different UEs(ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-0723 15.zip)    Non-Patent Document 3: Support of ACK Repetition for E-UTRA Uplink(ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—50/Docs/R1-0732 61.zip)